There is presently an increased demand for custom application-specific integrated circuit (ASIC) chips. As a result, production output requirements have increased.
One particular challenge associated with increasing production of ASIC chips is the testing that is required to sort out functional chips from non-functional chips. Specifically, each chip produced should be tested before packaging. However, with the sizes of the chips and associated components shrinking, testing becomes increasingly more difficult.
Probe stations typically use an array of tungsten wire probes to contact bond pads on the surface of each chip. These probes are held stationary and the chips to be tested are raised until the probes make proper electrical contact with the bond pads. These probes are used to apply the necessary power, ground and signal inputs and outputs to test the functions of a chip.
In order to ensure that a proper electrical contact is made between the bond pad and the probe, it is standard practice to employ a “scrubbing” motion with the probes when contacting the bond pad. This motion, however, causes damage to the bond pad surface, typically rendering the damaged portions unusable.
As chip sizes decrease, bond pad sizes also decrease. As a result, the marks left by the probes will consume an ever larger fraction of the bond pad surface.
In order to reduce the size and damage of the scrubs marks, probing parameters such as probe force, probe velocity and overdrive have been varied. Unfortunately, a trade-off exists between finding the probe parameters that minimize the size and damage of the scrubs marks and the low contact resistance, e.g., as achieved by a greater probe force, needed to perform proper chip testing. In fact, it has been found that current probe technology and parameter variations can no longer provide sufficiently low contact resistance while at the same time providing low scrub damage.
It is known that using a copper alloy material, such as aluminum-copper alloy, to form the bond pad will, to some degree, improve the electrical contact resistance properties of the bond pad. Therefore, bond pads typically comprise aluminum alloyed with small amounts, e.g., less than or equal to about two atomic percent (at. %), of copper. The amount of copper, however, has to be limited to these small amounts because its presence adversely affects the fabrication of the bond pad. Specifically, higher levels of copper adversely affect dry etch processes typically employed to create the bond pad (making it difficult to properly define various features). Further, bulk copper addition to the bond pad during fabrication can adversely affect the mechanical properties of the bond pad. For example, copper concentrations greater than four percent can cause a decrease in the yield strength (a measure of the force required to cause a plastic deformation of the material) of the bond pad material.
Therefore, techniques for bond pad fabrication that result in robust bond pads, with high yield strength, that sustain little, if any, damage during testing, yet maintain a low contact resistance would be desirable.